Why is NYC's Freedom Tower making this moaning noise?

People have been passing around this video for the past few days because it captures an astonishing phenomenon. The newly-built Freedom Tower emits a long, flute-like moaning sound. It's eerie and it's real. But no, it's not the ghosts of 9/11. » 12/04/13 1:20pm 12/04/13 1:20pm